the biophysical basis of amyloid formation will be studied using recombinant Immunoglobulin light chain (LC) proteins as the model. LC's or their fragments are found in the most common form of amyloid seen in the US, AL. The primary structural features, chemical and biochemical factors which alter LC secondary and tertiary structure, leading to fibril deposition, are undefined. The hypothesis to be tested is that AL fibril deposition occurs as a result of amino acid substitutions in critical regions of the LC, destabilizing them, resulting in insolubility. AL is a plasma cell dyscrasia and usually results in overproduction of a monoclonal protein in serum and urine. The prognosis for patients with major organ system involvement is quite poor with death resulting within months. The Specific Aims are: (1) Recombinant amyloidogenic and non-amyloidogenic LC's of the kappa 1 sub-class will be produced in E. Coli. The primary sequences of these LC's are based on clinically-derived proteins. (2) Their biophysical properties will be investigated to test their stability and amyloid-forming tendencies under controlled conditions of temperature, pH, ionic strength, salt type, protein concentration, and interaction with other fibril-associated proteins, such as P-component, glycosaminoglycans, and apoprotein E. Circular dichroism and high resolution calorimetry will be used to study the structure and conformation, thermodynamic and thermal stability of soluble LC. Light and electron microscopy and differential scanning calorimetry will be used to investigate the morphology and thermal stability of insoluble LC fibrils. The monomer-aggregate-fibril conversion will be observed chromatographically and spectroscopically. (3) New LC's will be designed using data obtained in (2) and molecular modeling of known LC structures. These LC's will be produced and their biophysical properties examined. Our studies on recombinant and clinically-derived LC's of the kappa 1 sub-class will correlate structural properties with functional characteristics of protein folding, providing quantitative data on factors which destabilize LC structure, identifying critical features which render LC's amyloidogenic. The biophysical basis and mechanism of amyloid deposition may be further defined and therapies for prevention designed.